1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine, more specifically to an exhaust gas purification system for an internal combustion engine comprising an NOx (Nitrogen Oxide) adsorber catalyst which absorbs NOx when exhaust gas inflow is lean in its air fuel ratio, and releases NOx when the concentration of oxygen of exhaust gas inflow becomes low.
2. Description of Related Art
It has been conventionally known, to lessen the content of NOx in exhaust gas discharged from an internal combustion engine (e.g., diesel engine), an exhaust gas purification system is introduced in connection with the internal combustion engine.
The exhaust gas purification system comprises a NOx adsorber catalyst which absorbs NOx when exhaust gas inflow is lean in its air fuel ratio, and releases NOx when the concentration of oxygen of exhaust gas inflow becomes low, the NOx adsorber catalyst being placed in an exhaust passage from the internal combustion engine.
The exhaust gas purification system includes one proposed in the Japanese Patent Publication No. 2586739.
This exhaust gas purification system comprises an NOx adsorber catalyst as above; an NOx mass estimating means for estimating the mass of NOx absorbed by the NOx adsorber catalyst; and a release means which, whenever the mass of NOx the estimating means estimates to be absorbed by the NOx adsorber catalyst exceeds a predetermined value, causes the NOx adsorber catalyst to release NOx. With this system, the release means causes the NOx adsorber catalyst to release NOx by lowering the oxygen concentration of exhaust gas inflow to be in contact with the NOx adsorber catalyst after having increased the supply of fuel to the cylinder of the internal combustion engine so as to make the engine run nearly on stoichiometric air fuel ratio.
According to the exhaust gas purification system as described above, whenever the NOx mass the NOx mass estimating means estimates to be absorbed by the NOx adsorber catalyst exceeds a predetermined value, the release means, regardless of the current load of the internal combustion engine, causes the NOx adsorber means to release NOx by increasing the fuel supply to the cylinder.
In other words, the release means increases the fuel supply to the cylinder as long as the same criterion is met, regardless of whether the engine works at a high load range or at a low load range.
However, when the internal combustion engine works at a high load range, the pressure within the cylinder (cylinder pressure) is high. Thus, if the amount of fuel supplied to the cylinder which is in such a sate as above is increased, the temperature within the cylinder will become considerably high. Because of this, if such an episodic increase of fuel supply to the cylinder is frequently repeated while the internal combustion engine runs in a high load range, it will encroach on the durability of the engine itself.
Particularly, if the internal combustion engine is equipped with a super charger, the amount of air supplied to the cylinder becomes so high that the temperature within the cylinder will be elevated which may further affect the durability of the engine. Moreover, then, the amount of air discharged from the cylinder will also become so high that, to keep the air fuel ratio close to stoichiometric value, a large amount of fuel must be supplied to the cylinder, which will lower the engine""s rate of fuel consumption, and worsen the driving cost.
This invention has following features, in order to provide an exhaust gas purification system for an internal combustion engine by which it is possible not only to reduce the NOx content of exhaust gas, but also to restrict aggravation of the durability and the rate of fuel consumption of the internal combustion engine to a minimum limit.
In one aspect, this invention provides an exhaust gas purification system for an internal combustion engine being equipped with an NOx adsorber catalyst which absorbs NOx when an exhaust gas lean in the air fuel ratio flows in, and releases NOx when the oxygen concentration of exhaust gas inflow becomes low comprising an NOx mass estimating means for estimating the mass of NOx depositing in the NOx adsorber catalyst; a first air fuel ratio controlling means which, whenever the NOx deposition estimated by the NOx mass estimating means reaches a predetermined value (reference NOx level), causes the NOx adsorber catalyst to release NOx by altering the air fuel ratio of exhaust gas coming in contact with the NOx adsorber catalyst to a value close to stoichiometric one; and a reference NOx level varying means which varies the reference NOx level according to the load and engine speed (rate of revolution) of the internal combustion engine.
According to this invention, the first air fuel ratio controlling means changes the air fuel ratio of exhaust gas from the internal combustion engine to a value close to stoichiometric one, and lowers the concentration of oxygen of an exhaust gas coming in contact with the NOx adsorber catalyst, thereby causing the latter to release NOx. Specifically, the means in question causes the catalyst to release NOx deposited thereupon as N2 which contributes to the lowered concentration of NOx of exhaust gas.
The first air fuel ratio controlling means, whenever the NOx deposition (the amount of NOx accumulated in the NOx adsorber catalyst) estimated by the NOx mass estimating means reaches a predetermined value (reference NOx level), alters the air fuel ratio of an exhaust gas coming in contact with the NOx adsorber catalyst to a value close to stoichiometric one, and the reference NOx level is also varied by the reference NOx level varying means in accordance with the load of the internal combustion engine, or with the load and engine speed of the internal combustion engine. If, for example, the engine runs with a high load, or with a high load at a high engine speed, that is, the amounts of fuel and air supplied to a cylinder of internal combustion engine are high, the reference NOx level may be set high. Then, it will be possible to lessen the number of occasions in which the air fuel ratio of exhaust gas must be altered to a value close to stoichiometric one which would be otherwise required when the internal combustion engine works at a high load range. This will restrict aggravation of the rate of fuel consumption and durability of the internal combustion engine to a minimum limit.
With this invention, the reference NOx level setting means may preferably set the reference NOx level close to the maximum attainable level of NOx deposition for the NOx adsorber catalyst when the engine runs at a high speed in a high load range, while it may preferably set the reference NOx level at a level lower than the maximum attainable level of NOx deposition for the NOx adsorber catalyst when the engine runs at a low speed in a low load range.
According to this invention, when the internal combustion engine runs in a high load range where the engine would be impaired in its durability and rate of fuel consumption if recourse to the driving at about stoichiometric air fuel ratio were frequently introduced, the reference NOx level setting means sets the reference NOx level close to the maximum attainable level of NOx deposition for the NOx adsorber catalyst, and thus can reduce to a least possible limit such frequent recourse to the driving at about stoichiometric air fuel ratio, thereby restricting aggravation of the durability and rate of fuel consumption of the internal combustion engine to a minimum limit.
In contrast, when the engine runs in a low load range, the means in question sets the reference NOx level below the maximum attainable level of NOx deposition for the NOx adsorber catalyst, and thus allows the NOx adsorber catalyst to keep a room to accept the deposition of new NOx. Through this arrangement, the need for the immediate alteration of the reference NOx level to stoichiometric value can be effectively avoided, even when the engine""s run shifts from a low load range to a high load range where emission of NOx is increased.
With this invention, the reference NOx level setting means may preferably set the reference NOx level to a level approximately equal to the maximum attainable level of NOx deposition for the NOx adsorber catalyst when the internal combustion engine runs at a high speed in a high load range, while the same means may preferably set the same level below the same maximum attainable level when the engine runs at a low speed in a low load range.
According to this invention, when the internal combustion engine runs at a high speed in a high load range where the engine would be seriously impaired in its durability and rate of fuel consumption if recourse to the driving at about stoichiometric air fuel ratio were frequently introduced, the reference NOx level setting means sets the reference NOx level close to the maximum attainable level of NOx deposition for the NOx adsorber catalyst, and thus can maintain at a least possible limit such frequent recourse to the driving at about stoichiometric air fuel ratio, thereby restricting aggravation of the durability and rate of fuel consumption of the internal combustion engine to a least possible limit.
In contrast, when the engine runs at a low speed in a low load range where frequent recourse to the driving at about stoichiometric air fuel ratio is less burdensome, the reference NOx level setting means sets the reference NOx level below the maximum attainable level of NOx deposition for the NOx adsorber catalyst, and thus allows the NOx adsorber catalyst to keep a room to accept the deposition of new NOx. Through this arrangement, the need for the immediate alteration of the reference NOx level to stoichiometric value can be effectively avoided, even when the engine""s run changes from a low speed/low load range to another range including a high speed/high load range.
With this invention, when the engine runs in a low load range or in a low speed/low load range, it is preferable to set the reference NOx level appropriately depending on the current driving mode of the engine.
For example, if the driving mode consists of running the engine with a high load at one time and a low load at another at regular intervals, shifts from the low load driving to the high load driving and vice versa become more frequent and the duration at which the low load driving is maintained becomes shorter than are observed in a driving mode where the engine runs constantly in a low load range.
According to this invention, when the internal combustion engine runs in a low load range (or low speed/low load range), the reference NOx level setting means sets the reference NOx level appropriately in accordance with the current driving mode of the engine. Thus, for example, if the current driving mode consists of a series of brief running at a low load range (low speed/low load range), the reference NOx setting means sets the reference NOx level below a level which is usually introduced for the normal low load running, so as to cause the NOx adsorber catalyst to release NOx deposited thereupon at as low a level as possible. This will serve to maintain the frequency of recourse to the driving at about stoichiometric air fuel ratio whenever the engine enters into a high load range, at a least possible limit.
With this invention, when the engine runs in a low load range or in a low speed/low load range, the reference NOx level setting means may preferably set the reference NOx level at a level equal to one third to two thirds of the maximum NOx deposition attainable for the NOx adsorber catalyst.
According to this invention, when the internal combustion engine runs with a low load (in a low speed/low load range), the reference NOx level setting means sets the reference level at one third to two thirds of the maximum NOx deposition attainable for the NOx adsorber catalyst, and thus allows the NOx adsorber catalyst to keep a room sufficiently wide to accept new NOx equal in amount to the two thirds to one third of its maximum attainable NOx deposition. Through this setting, the need for the immediate alteration of the reference NOx level to stoichiometric value can be effectively avoided, even when the engine""s run shifts from a low load range (or low speed/low load range) to a high load range (or to another range including a high speed/high load range).
With this invention, the system may preferably include a engine speed detecting sensor to detect the engine speed of the internal combustion engine, and a load detecting sensor to detect the load of the internal combustion engine, and the NOx mass calculating means may preferably calculate the mass of NOx deposited onto the NOx adsorber catalyst based on the information provided by the engine speed detecting sensor and load detecting sensor.
According to this invention, because the NOx mass calculating means calculates the NOx deposition on the NOx adsorber catalyst based on signals delivered from the engine speed detecting and load detecting sensors routinely attached to the internal combustion engine, need for the separate installment of an NOx sensor for determining the NOx deposition can be effectively avoided, which helps reduce the production cost of the exhaust gas purification system of this invention.
With this invention, preferably a second air fuel ratio controlling means may be introduced which causes the NOx adsorber catalyst to release NOx by altering the air fuel ratio of exhaust gas discharged from the internal combustion engine to come into contact with the NOx adsorber catalyst, immediately after the engine""s run shifts from a high load range to a low load range, or from a high speed/high load range to a low speed/low load range.
According to this invention, whenever the engine""s run shifts from a high load to a low load range, or a high speed/high load to a low speed/low load range, the second air fuel ratio controlling means alters the air fuel ratio of exhaust gas from the internal combustion engine close to stoichiometric value.
Because of this, whenever the engine""s run shifts from a high load range (high speed/high load range) where the temperature/pressure within the cylinder as well as emission of NOx are high, to a low load range (low speed/low load range), the second air fuel ratio controlling means will ensure the release of NOx from the NOx adsorber catalyst, thereby securely lessening the NOx deposition on the catalyst no matter how much NOx has deposited thereupon. Therefore, even if the engine""s run swiftly turns from the low load range back to a high load range, the original run may be continued for a while and the need for immediate recourse to the driving at about the theoretical air fuel ratio can be safely avoided.
With this invention, a air inflow restricting means may be introduced which is capable of restricting the inflow of air to the cylinder of the internal combustion engine, and the first and/or second air fuel ratio controlling means may preferably alter the air fuel ratio of exhaust gas from the engine to stoichiometric value by restricting air inflow to the cylinder via the air inflow restricting means.
According to this invention, because the air inflow restricting means capable of restricting the air inflow to the cylinder is introduced, it is possible to easily alter the air fuel ratio of exhaust gas. For example, if the air fuel ratio is in a lean range, the air inflow restricting means may be adjusted to lessen air inflow to the cylinder, which enables the air fuel ratio of exhaust gas to be close to stoichiometric value without seriously affecting the rate of fuel consumption of the engine.
With this invention, the internal combustion engine may preferably have a supercharger equipped with a compressor, and the air inflow restricting means may be configured in such a manner as to enable part of air accumulated on the compressor""s outlet attached to the super charger to be vented towards the downstream of the NOx adsorber catalyst.
According to this invention, because the air inflow restricting means is introduced which is capable of diverting part of feed air accumulated on the compressor""s outlet of super charger towards the downstream of the NOx adsorber catalyst, it is possible to ensure the driving at stoichiometric air fuel ratio without seriously affecting the rate of fuel consumption of the engine which would otherwise result when air inflow to the engine with a super charger is lessened with no proper care.
Further, because part of feed air is diverted to the downstream of the NOx adsorber catalyst under the feed pressure of the super charger, the part is securely exhausted without being reflected back.
With this invention, an operation monitoring means for monitoring the operation of the air inflow restricting means, and a fuel feed correcting means for correcting the amount of fuel fed to the cylinder of internal combustion engine based on a signal sent by the operation monitoring means may be preferably introduced.
According to this invention, operation of the air inflow restricting means is monitored by the operation monitoring means, and the amount of fuel fed to the cylinder is corrected based on a signal sent by the operation monitoring means. Through this arrangement, it is possible, when the air inflow restricting means is midway in operation for shifting the air fuel ratio from a lean range to about a theoretical level, or from about a theoretical level to a lean range, to adjust the amount of fuel fed to the cylinder in accordance with the current operation state, and thus to achieve the proper setting of the air fuel ratio even when the air inflow restricting means is midway in its operation.